Output Coupler Research Articles

Passively Q‐switched Tm:YAG ceramic laser with TiS2 saturable absorber

AbstractA two‐dimensional titanium disulfide (TiS2) nanosheet film is prepared on a YAG crystal substrate. The saturated absorption behavior of TiS2 on the YAG substrate is investigated. Then, TiS2 is successfully employed as a modulation element to develop a Tm:YAG ceramic eye‐safe pulsed laser. With a 7.04 W absorbed pump light and a 3% transmission of output coupler, the greatest average optical power, and the least pulse width (PD) are 233 mW and 524 ns, separately. The pulse repeated frequency of 69 kHz is generated, while the single pulse energy (SPE) is 3.38 µJ. Based on the experimental results, TiS2 nanosheet films possess the practical value as saturable absorbers for 2.0 μm eye‐safe pulsed lasers.

Design and experiment of broadband electric field sensor for high-power sub-nanosecond pulse measurements

This paper briefly introduces the structure and working principle of the electric field sensor. And a conductive electric field sensor for measuring sub-nanosecond voltage pulse is designed. The sensor consists of coaxial transition section and integrated electric field probe. N-type connector and SMA connector are selected for input/direct output port and coupling output port respectively. The equivalent antenna area of electric field sensor measured by vector network analyzer is about 0.035cm2, and the 3dB bandwidth is more than 5GHz. The results show that the sensor can accurately measure the time-domain voltage pulse produced by the sub-nanosecond solid-state sources.

Breadboard of Microchip and Avalanche Photodiode in Linear and Geiger Mode for LiDAR Applications

This paper reports the implementation of two critical technologies used in LiDARs: 1) A microchip Q-switched laser breadboard and 2) breadboard of an Indium gallium arsenide avalanche photodiode working at 300 K with high reverse polarization voltages. Microchip Q-switched lasers are small solid state back pumped lasers, that can generate high energy short pulses. The implemented breadboard used an Erbium and Ytterbium co doped phosphate glass, a COMALO crystal with 98% (initial transparency) and an output coupler of 98% reflectivity. For the sensor test, a system for the simultaneous operation in vacuum and wide range of temperatures was developed. Avalanche photodiodes are reverse polarized photodiodes with high internal gain, due to their multiple layer composition, capable of building up high values of photocurrent from small optical signals by exploiting the avalanche breakdown effects. The test avalanche photodetector was assembled to be operated in two modes: Linear and Geiger mode, to achieve this behavior, a transimpedance amplifier circuit was implemented. These two technologies are critical for mobile LiDAR applications, due to its low mass and high efficiency. The paper describes the breadboard implementation method and sensor characterization at low temperature and high voltage (beyond breakdown voltage).

A Modified Double Staggered Grating Waveguide Slow Wave Structure for Sub-THz Traveling Wave Tubes

A modified double staggered grating waveguide (DSGW) slow wave structure (SWS) is proposed for millimeter-wave and sub-THz traveling wave tubes (TWTs). Gratings with a cross section outline of exponential curves are applied instead of the traditional rectangular cross section. The proposed SWS owns a higher interaction impedance compared to DSGW, resulting in higher output power and gain. With proper design and optimization of the input–output couplers, the proposed SWS can have a wide <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 15 dB impedance bandwidth, extending from 192 to 230 GHz. Particle-in-cell (PIC) simulations are carried out for a TWT with sheet electron beam (SEB) and the main SWS having 70 periods. Such a TWT provides a maximum power of 116.64 W which indicates a significant improvement compared with the traditional DSGW.

Inline Box-Like Dielectric Filters With Asymmetric and Symmetric Responses

In this article, a new class of inline filters with box-like properties and pseudoelliptic responses are presented. The proposed filters employ dual-ridge dual-mode resonators (DRDMRs). Each DRDMR can be used in modular filter design as a basic building block, which provides two poles and a transmission zero (TZ). Asymmetric and symmetric filtering responses can be obtained by simply changing the resonant frequencies of the DRDMRs while all coupling coefficients remain unchanged in both values and signs. By properly arranging the position of the coaxial feeding probe, the input–output coupling of the two resonant modes can be controlled flexibly and thus control the TZ position. Intracavity coupling is realized by the all-iris structure of the dual-mode form. To validate the feasibility, an inline fourth-order filter with a pair of symmetric TZs has been designed and manufactured. The experimental result agrees well with the simulated result.

A broadband SPR sensor based on a no-core fiber coated with gold-silver for refractive index and temperature measurement

Dual-parameter sensor has relatively small detection range because of restriction on resonance wavelength range. To overcome this problem, we propose a cascaded gold and silver SPR dual-parameter sensor. As the resonance wavelength ranges of gold and silver are different, cascading the two metals to make one sensor broadens the resonance wavelength range, thus increasing the sensor's detection range. The sensor uses no-core fiber as the probe and multimode fiber (MMF) as the input (output) coupler, which constitutes an MMF-NCF-MMF sensing structure. By using Magnetron sputtering, gold and silver are deposited on the outer surface of NCF. The effects of coating time and NCF length on the sensing performance were experimentally investigated to determine the optimal coating time and NCF length. The experimental results shows that the two channels of the sensor are independent of each other without crosstalk and have an extensive range of parameters to be measured. The maximum sensitivity is 3231 nm/RIU in the RI range of 1.333–1.385. It has a maximum temperature sensitivity of 6.68 nm/°C from 0 to 80 °C. In addition, the sensor has good stability and repeatability. The proposed sensor has a simple and integrated structure and is suitable for applications requiring large detection range sensing.

Assessment of possible performances of compact solar-pumped lasers using transparent Cr3+, Nd3+-co-doped YAG ceramics micro rods

Employing a commercially available off-axis parabolic mirror of ⌀50.8 or ⌀76. 2 mm caliber and ⌀1 or 1 × 1 × 10 mm Cr3+, Nd3+-co-doped YAG ceramic micro laser medium (LM), our compact solar-pumped lasers (μSPL) could oscillate continuously without any forced air cooling from 11:00 a.m. beyond 5:30 p.m. tracking the sun outdoors. However, their energy conversion efficiencies ηpower have not exceeded so far 1%. Numerical assessments of μSPL performance using a fundamental expression of laser oscillation based on experimentally obtained parameters were performed. Dependences of ηpower on transmittance of an output coupler, mode-matching efficiency ηmode, length of LM, distributed loss constant in LM, and Cr content as well as the necessity of the concentration ratio as high as 10,000 were elucidated. The values of ηmode between 0.3 and 0.5 coming from the present confocal resonator roughly reproduced ηpower around 1%. Based on the assessments, possible approaches for further improvement of μSPL performance were evaluated including employment of composite LM to increase ηmode.

Maximum Pump Power Coupled in Raman Resonator for Maximum Power Delivered at 1115 and 1175 nm

In this report, we present our analysis of the relationship between critical power and stimulated Raman scattering in Raman fiber lasers. Through our research, we have established a connection between the R.G. Smith constant at critical power and the necessary pump power required to reach the maximum power delivered by the first Stokes just prior to the generation of the second Stokes. In our experiments, two setups were successful in reaching the second Stokes generation, one utilizing a glass–air interface as the output coupler without HR mirrors and the other using HR-FBGs for both Stokes in conjunction with a glass–air interface. We found that the 1 Km 1060-XP fiber has an R.G. Smith constant of ~4.94 at critical power, which when multiplied by 2 gives ~9.88, a value close to the R.G. Smith constant (9.75) for maximum Stokes corresponding to a pump power of 5.5 W, with an approximation of ~98.6%. Our results demonstrate the importance of knowing the R.G. Smith constant at critical power in estimating the necessary pump power to achieve maximum power delivery in any Stokes component.

2.7 μm mid-infrared Er-doped thin-film LiNbO3-on-insulator photonic wire laser

The 2.7 μm continuous-wave laser was demonstrated theoretically on the basis of a 1480-nm-pumped Er-doped thin-film LiNbO3-on-insulator photonic wire (Er:LNOI PW) and a three-level rate equation model of Er3+. Under 1480 nm pumping, the excited state absorption (4I13/2 → 4I9/2) and cooperative upconversion process (4I13/2 → 4I15/2: 4I13/2 → 4I9/2) populate the 4I11/2 level meanwhile bypass significantly the 4I13/2 level, enabling a population inversion between these two levels. The cavity length and output coupler's reflectivity at laser wavelength were optimized for maximizing laser output. Under the optimum configuration, dependences of laser power on both pump power and Er3+ concentration were investigated. Simulation results show that lower threshold pump power, higher laser output and slope efficiency are obtained for a higher Er3+ concentration. The laser output reveals a linear relationship to the Er3+ concentration and the relationship is strong at higher pump level. Simulation results show that the threshold pump power, laser output and slope efficiency are on orders of several tens of mW, sub-mW and 3.1%, respectively, for an Er3+ concentration of 1.5 mol%. In addition, there is no need to account for photorefractive effect on the laser performance as it is far smaller than the refractive index contrast of the LNOI PW, ∼0.7. Present work opens up the possibility of implementing an on-chip 2.7 μm mid-infrared laser based on an Er:LNOI.

1.8-µm laser operation based on femtosecond-laser direct written Tm:YVO4 cladding waveguides.

In this work, we have demonstrated tunable 1.8-µm laser operation based on a Tm:YVO4 cladding waveguide fabricated by means of femtosecond laser direct writing. Benefiting from the good optical confinement of the fabricated waveguide, efficient thulium laser operation, with a maximum slope efficiency of 36%, a minimum lasing threshold of 176.8 mW, and a tunable output wavelength from 1804 to 1830nm, has been achieved in a compact package via adjusting and optimizing the pump and resonant conditions of the waveguide laser design. The lasing performance using output couplers with different reflectivity has been well studied in detail. In particular, due to the good optical confinement and relatively high optical gain of the waveguide design, efficient lasing can be obtained even without using any cavity mirrors, thereby opening up new possibilities for compact and integrated mid-infrared laser sources.

Breadboard of Microchip Laser and Avalanche Photodiode in Geiger and Linear Mode for LiDAR Applications

This paper reports the implementation of two critical technologies used in light detection and ranging for space applications: (1) a microchip Q-switched laser breadboard; (2) a breadboard of an indium gallium arsenide avalanche photodiode working at 292 K with high reverse polarization voltages. Microchip Q-switched lasers are small solid-state back-pumped lasers that can generate high-energy short pulses. The implemented breadboard used an erbium and ytterbium co-doped phosphate glass, a Co:Spinel crystal with 98% initial transparency, and an output coupler with 98% reflectivity. For the sensor test, a system for simultaneous operation in vacuum and a wide range of temperatures was developed. Avalanche photodiodes are reverse-polarized photodiodes with high internal gain due to their multiple layer composition, capable of building up high values of photocurrent from small optical signals by exploiting the avalanche breakdown effects. The test avalanche photodetector was assembled to be operated in two modes: linear and Geiger mode. The produced photocurrent was measured by using: (1) a passive quenching circuit; (2) a transimpedance amplifier circuit. These two technologies are important for mobile light detection and ranging applications due to their low mass and high efficiencies. The paper describes the breadboard’s implementation methods and sensor characterization at low and room temperatures with high bias voltages (beyond breakdown voltage).

Red, orange, and dual wavelength vortex emission from Pr:WPFGF fiber laser using a microscope slide output coupler.

Visible vortex beams have a large array of applications; however, the sources are often large or complex. Here, we present a compact vortex source with red, orange, and dual wavelength emission. This Pr:Waterproof Fluoro-Aluminate Glass fiber laser uses a standard microscope slide as an interferometric output coupler, yielding high quality first order vortex modes in a compact setup. We further demonstrate the broad (∼5 nm) emission bands in the orange (610 nm), red (637 nm) and near-infrared regions (698 nm), with the potential for green (530 nm) and cyan (485 nm) emission. This is a low-cost, compact and accessible device giving high quality modes for visible vortex applications.

Divisional Load-Modulated Balanced Amplifier With Extended Dynamic Power Range

With the prevalence of complex modulated signals, it is of utmost significance to research the extension of the dynamic power range of radio frequency (RF) power amplifiers (PAs). This article proposes a divisional load-modulated balanced amplifier (DLMBA) for extending the high-efficiency power range. The proposed DLMBA is composed of a local modulated Doherty PA (DPA) and a class-C biased balanced PA (BPA) pair. The output signal of the local DPA is fed into the isolation port of the output coupler of the BPA pair, forming a global load modulation. The design parameters of the DLMBA are derived after a comprehensive analysis. It is illustrated that the dynamic power range of the DLMBA can be extended beyond 12 dB by combining a local DPA and a BPA pair. Meanwhile, each subamplifier of the DLMBA is presented with a modulated impedance over the whole dynamic power range, avoiding voltage oversaturation. As a validation, a DLMBA operating over 1.85–2.05 GHz is fabricated and measured in this article. The fabricated DLMBA delivers a maximum output power of more than 44.8 dBm, a saturation drain efficiency (DE) of more than 53%, and a 13 dB back-off DE of more than 44% in the interested frequency band. Meanwhile, when the DLMBA is excited by a 20-MHz wideband signal with a peak-to-average ratio of 10.2 dB, the measured adjacent channel leakage ratio reaches <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 47.4 dBc after digital predistortion.

Compact Waveguide Filters Using Novel Resonant Coupling Structures

In this article, new resonant coupling structures are proposed for designing miniaturized waveguide bandpass filters. The resonant coupling structure is constructed by adding one or two metal blocks in a relatively thick inductive iris, which can be considered as the integration of a resonator, the input, and output coupling structures. Positive or negative coupling can be realized by selecting a different number of metal blocks. The combination of different resonant coupling structures can be used to design higher-order filters with various transverse topologies, realizing different transmission zeros (TZs) distributions. In addition, the weak source–load coupling caused by the resonant coupling structures can introduce an additional TZ, which can further improve the out-of-band suppression level of the filter. For verification, two third-order waveguide bandpass filters based on all resonant coupling structures are designed and simulated to achieve different TZ distributions. Finally, a fifth-order waveguide bandpass filter employing both resonant coupling structures and classical rectangular cavities is designed and fabricated. The measured results verify the proposed method.

Diuretics depletion improves cardiac output and ventriculo-arterial coupling in congestive ICU patients during hemodynamic de-escalation.

Congestion was shown to hamper organ perfusion, but the exact timing of diuretic initiation during hemodynamic de-escalation in shock is unclear. The aim of this study was to describe the hemodynamic effects of diuretic initiation in the stabilized shock. We performed a monocentric, retrospective analysis, in a cardiovascular medico-surgical ICU. We included consecutive resuscitated adult patients, for whom the clinician decided to introduce loop diuretic treatment for clinical signs of fluid overload. The patients were hemodynamically evaluated at the moment of diuretic introduction and 24h later. Seventy ICU patients were included in this study, with a median duration of ICU stay before diuretic initiation of 2 [1-3]days. 51(73%) patients were classified as congestive (central venous pressure > 12 mmHg). After treatment, the cardiac index increased towards normal values in the congestive group (2.7 ± 0.8L min- 1m- 2 from 2.5 ± 0.8L min- 1m- 2, p = 0.042), but not in the non-congestive group (2.7 ± 0.7L min- 1m- 2 from baseline 2.7 ± 0.8L min- 1m- 2, p = 0.968). A decrease in arterial lactate concentrations was observed in the congestive group (2.1 ± 2 mmol L- 1 vs. 1.3 ± 0.6 mmol L- 1, p < 0.001). The diuretic therapy was associated with an improvement of ventriculo-arterial coupling comparing with baseline values in the congestive group (1.69 ± 1 vs. 1.92 ± 1.5, p = 0.03). The norepinephrine use decreased in congestive patients (p = 0.021), but not in the non-congestive group (p = 0.467). The initiation of diuretics in ICU congestive patients with stabilized shock was associated with improvement of cardiac index, ventriculo-arterial coupling, and tissue perfusion parameter. These effects were not observed in non-congestive patients.

Design optimization of polarization volume gratings for full‐color waveguide‐based augmented reality displays

AbstractReflective polarization volume gratings (PVGs) can be used as input and output couplers in a diffractive waveguide‐based augmented reality (AR). To improve the stability and wearing comfort, a thin and lightweight full‐color waveguide display is needed. In this paper, we propose a single full‐color waveguide AR display based on PVGs. By analyzing the optical system, we derived the field of view limit by optimizing a three‐layer PVG as an in‐coupler. Pairing with a single‐layer, thin PVG as an out‐coupler, we can achieve a relatively uniform exit pupil of 8 mm at horizontal pupil expansion direction.

Single-image-source binocular waveguide display based on polarization volume gratings and lenses.

Waveguide displays, a highly competitive solution for augmented reality (AR), have attracted a lot of interest. A polarization-dependent binocular waveguide display using polarization volume lenses (PVLs) and polarization volume gratings (PVGs) as input and output couplers, respectively, is proposed. Light from a single image source is delivered to the left and right eyes independently according to its polarization state. Compared with traditional waveguide display systems, no additional collimation system is needed due to the deflection and collimation capabilities of PVLs. Leveraging the high efficiency, wide angular bandwidth, and polarization selectivity of liquid crystal elements, different images can be independently and accurately produced in the two eyes when the polarization of the image source is modulated. The proposed design paves the way for a compact and lightweight binocular AR near-eye display.

Topology Optimization Design of Multi-Input-Multi-Output Compliant Mechanisms with Hinge-Free Characteristic and Totally Decoupled Kinematics

A new multi-constraint optimization model with the weighted objective function is proposed to design the multi-input-multi-output (MIMO) compliant mechanisms. The main feature of this work is that both the two notable problems related to the de facto hinge and the movement coupling are tackled simultaneously in the topological synthesis of MIMO compliant mechanisms. To be specific, the first problem is the severe stress concentration in the flexible hinge areas, and it is solved by the introduction of input and output compliances into the objective function, which could facilitate the optimization to strike a good balance between structural flexibility and stiffness. The second problem is the high degree of control complexity caused by the coupled outputs and inputs, and it is addressed by achieving the complete decoupling with two groups of extra constraints that are used to suppress the input coupling and the output coupling, respectively. As the most common and effective topology optimization method, the Solid Isotropic Material with Penalization (SIMP)-based density method is adopted here to obtain the optimized configurations. After the analytical sensitivity deduction related to the weighted objective function and constraints, two typical numerical examples are presented to demonstrate the validity of the proposed topology optimization framework in designing the hinge-free and completely decoupled MIMO compliant mechanisms.

Adaptive fixed-time output synchronization for complex dynamical networks with multi-weights

This paper addresses fixed-time output synchronization problems for two types of complex dynamical networks with multi-weights (CDNMWs) by using two types of adaptive control methods. Firstly, complex dynamical networks with multiple state and output couplings are respectively presented. Secondly, several fixed-time output synchronization criteria for these two networks are formulated based on Lyapunov functional and inequality techniques. Thirdly, by employing two types of adaptive control methods, fixed-time output synchronization issues of these two networks are dealt with. At last, the analytical results are verified by two numerical simulations.

Radiation-balanced lasing in Yb3+:YAG and Yb3+:KYW.

Radiation-balanced lasing and thermal profiling is reported in two Yb-doped laser crystals at room temperature. In 3% Yb3+:YAG a record efficiency of 30.5% was achieved by frequency-locking the laser cavity to the input light. Both the average excursion and axial temperature gradient of the gain medium were maintained within 0.1 K of room temperature at the radiation balance point. By including saturation of background impurity absorption in the analysis, quantitative agreement was obtained between theory and the experimentally measured laser threshold, radiation balance condition, output wavelength, and laser efficiency with only one free parameter. Radiation-balanced lasing was also achieved in 2% Yb3+:KYW with an efficiency of 2.2% despite high background impurity absorption, losses from Brewster end faces that were not parallel, and non-optimal output coupling. Our results confirm that relatively impure gain media can be operated as radiation-balanced lasers, contrary to earlier predictions which ignored background impurity properties.